1. Field of the Invention
The present invention relates to a passive optical network (PON), and more particularly, to a passive optical network system providing simultaneously both broadcasting service and data service.
2. Description of the Related Art
With the rapid increase of the number of Internet subscribers, the variety of Internet services, the appearance of large capacity contents, etc., subscribers' demand for a large capacity of data services is exponentially increasing. However, the conventional networks based on copper wires have limitations with respect to transmission speed and the integrity of data, so that it is not possible to satisfy the demand for data services by subscribers today and in the future.
Meanwhile, due to the development of electronic elements in recent years, the optical communication systems can transmit data at speeds of several tens of Tbps or more. Therefore, in order to provide data services at high speed and with good integrity, the construction of a FTTH (Fiber To The Home) system, which connects optical fibers directly to the subscriber terminals, is required. The FTTH system can provide the transmission speed and the integrity demanded for data services today and in the future. A passive optical network (hereinafter, referred to as “PON”) is being highlighted as a FTTH system, since the PON is one of the most efficient and economical networks from the viewpoint of its construction and maintenance.
FIG. 1 illustrates a conventional PON comprising a central office 10, an optical divider coupler 21, and a plurality of subscriber terminals 30, which are connected with one another by transmission optical fibers 40 and 50.
The central office 10 provides optical communication service for the subscriber terminals 30 through the optical divider coupler 21. That is, the optical divider coupler 21 is connected with the central office 10 and the subscriber terminals 30 through the transmission optical fibers 40 and 50, thus being able to transmit the optical communication service provided from the central office 10 to the subscriber terminals 30.
To this end, the central office 10 includes a downstream optical source 11 for providing an optical source needed in order to transmit data to the subscriber terminals 30 (downstream transmission), an upstream optical receiver 13 for receiving data transmitted upstream from the subscriber terminals 30, and a first wavelength division multiplexer 15 for filtering and transmitting the signals output from the downstream optical source 11 and the signals transmitted to the upstream optical receiver 13. The subscriber terminals 30 include an upstream optical source 31 for providing an optical source needed in order to upstream transmit data to the central office 10, a downstream optical receiver 33 for receiving data transmitted downstream from the central office 10, and a second wavelength division multiplexer 35 for filtering and transmitting the signals output from the upstream optical source 31 and the signals transmitted to the downstream optical receiver 33. In other words, the optical divider coupler 21 connects a central office 10 with the subscriber terminals 30.
Recently, a number of studies have been addressed to providing broadcasting service through a PON, which has historically been provided by a different hybrid fiber coaxial network (HFC network). That is because the PON may reduce the cost of constructing and maintaining the network for providing data service and broadcasting service simultaneously.
Meanwhile, the analog broadcasting system must have a high CNR (carrier to noise ratio) value so as to maintain clear screen quality on the subscriber terminals. Therefore, in constructing the network system for providing the broadcasting service by the PON, the optical signals being input into the optical receivers in the subscriber terminals must necessarily maintain a very high level of output so as to maintain a proper CNR.
By contrast, digital broadcasting in the future requires a relatively low CNR as compared with analog broadcasting, thus enabling the subscriber terminals to have clear screen quality, though the optical signals input into the optical receivers of the subscriber terminals have a relatively low level of output. However, in order to increase the capacity of the broadcasting channels, the output level must be increased of the optical signals being input into the optical receivers in the subscriber terminals in consideration of additional noise generated by the increased broadcasting signals, so that the optical signals may maintain a proper CNR.
Consequently, when the broadcasting service is provided by a PON, the PON must be constructed so as to provide optical signals of a high level output for the optical receivers in the subscriber terminals,
FIG. 2 illustrates a conventional PON for simultaneously providing both broadcasting service and data service and comprises, similar to the conventional PON illustrated in FIG. 1, a central office 10a, an optical divider coupler 21, and a plurality of subscriber terminals 30a, which are connected with one another by transmission optical fibers 40 and 50, and each component of equipment is also similar to those illustrated in FIG. 1.
FIGS. 1 and 2 differ only in that the central office 10a further comprises a broadcasting optical source 17 for a broadcasting service and an optical amplifier 19 for amplifying the signals output from the broadcasting optical source 17, and the subscriber terminals 30a further comprise a broadcasting optical receiver 37 for receiving the signals transmitted from the broadcasting optical source 17. The broadcasting optical source 17 modulates and outputs, according to the broadcasting signals, the optical signals of the wavelength which is distinguished from the wavelength of the optical signals outputted from the downstream optical source 11.
Therefore, a first wavelength division multiplexer 15a in the central office 10a multiplexes and outputs the optical signals, which are output from the broadcasting optical source 17 and amplified by the optical amplifier 19, and the optical signals output from the downstream optical source 11. Also, a second wavelength division multiplexer 35a in the subscriber terminals 30a demultiplexes the multiplexed optical signals as described above, and separates data service signals and broadcasting service signals. The separated data signals and broadcasting signals are provided for subscriber terminals after being data processed by the downstream optical receiver 33 and the broadcasting optical receiver 37, respectively.
As illustrated in FIG. 2, in order for a PON to simultaneously provide broadcasting service and data service, the conventional art directly amplifies the optical signals modulated according to the broadcasting signals by inputting them into the optical amplifier 19, so as to raise the output level of the optical signals being output for broadcasting service from the central office 10a, in order to raise the output level of optical signals being input into the broadcasting optical receiver 37 in the subscriber terminals 30a. 
However, when the optical signals for broadcasting service, having a higher output level than a basic value, are input to the transmission optical fiber 40, the noise component is rapidly raised by the nonlinear quality of optical fiber, so that it is impossible to maintain the proper CNR (carrier to noise ratio) required for the broadcasting service. Thus, the output level of the optical signals for broadcasting service, which is amplified by the optical amplifier 19 in the central office 10a and is input to the transmission optical fiber 40, is limited below a basic value by the nonlinear quality of optical fiber.
Therefore, in order to maintain the optical signals, which are input into the optical receivers (especially, the broadcasting optical receiver 37) in the subscriber terminals 30a, in the high level of the output, the number of branches of the optical divider coupler 21 must be reduced, thus it is possible to reduce the loss of the optical signals for broadcasting service.
However, since each PON determines the accessible number of the subscriber terminals according to the number of branches of the optical divider coupler 21, the number of subscriber terminals being accepted by the PON cannot help but be reduced when the broadcasting service is provided by the PON as a conventional art. For this reason, more PONs must be built. Consequently, to use the conventional PON it is more costly and time consuming due to the additional construction and maintenance.
Therefore, in order to efficiently provide the broadcasting service by a PON, it is necessary to develop a PON which is able to simultaneously provide both broadcasting service and data service without reducing the number of subscriber terminals.
Also, in order to increase the number of broadcasting channels for service according to the request of the subscriber, the conventional art must further increase the output level of the optical signals being input into the broadcasting optical receiver 37 of the subscriber terminals 30a because of the additional noise generated by the increased number of broadcasting channel signals, thus enabling the signals to be maintained within a proper CNR for clear screen quality. Therefore, it is necessary to develop a network system that can accept more broadcasting channels.